feat: dark mode for navbar and general layout background

[patricio0312rev]

[general] Dark Mode: navbar & general layout background

Summary

• General layout background styles have been updated to support dark mode.
• Same thing for Navbar component and its children components. (Send/Receive sidebars are still missing, since they are not part of this component).
• Button styles for primary, secondary and danger buttons/button-with-icons have been updated. Icon buttons still need work, since it has many variations/implementations. I ended up just updating the ones that we use for the navbar and menus.

NOTE: A switch has been added to Settings menu just for testing purposes. This should be removed before merging and implement the design/config properly once it is designed.

Steps to reproduce

1. Run the app in test_e2e or test mode.
2. Connect your wallet and go to Settings page.
3. Turn Dark mode toggle on and see the magic ✨

Checklist

• I checked my UI changes against the design and there are no notable differences
• I checked my UI changes for any responsiveness issues
• I checked my (code) changes for obvious issues, debug statements and commented code
• I opened a corresponding card on Clickup for any remaining TODOs in my code
• I added a short description on how to test this PR (if necessary)
• I added a storybook entry for the component that was added (if necessary)
• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[patricio0312rev]

I have some doubts/notes about this ticket:

• There are a lot of icon buttons, so even if I updated the _buttons.css file, some of them were missing (for example: Like buttons). This is the reason why I'm not marking buttons as done.

• The close button for mobile menu looks different in the icon section and in its implementation on design:
  • Icon section:

• Mobile menu implementation:

I am using the icon section styles, but let me know if you want it as its implementation.

• The Dark mode toggle I implemented on the settings page should be removed before merging this PR, since it is just for testing purposes.